1. Field of the Invention
This invention relates to switching regulators in general and, more particularly, to low output voltage switching regulators, typically referred to as "buck" regulators.
2. Description of the Prior Art
To meet the demand for faster and more powerful personal computers and workstations, microprocessor manufacturers are increasing the clock frequency at which the microprocessor operates. Because most of the microprocessors are implemented in complementary metal-oxide-semiconductor (CMOS) technology, the power dissipation of the microprocessor generally increases linearly with the clock frequency. In high power designs, the heat created by the power dissipated in the microprocessor is dissipated by fan-cooled heat sinks attached to the microprocessor's package. However, even this technique may be insufficient for dissipating sufficient heat with newer microprocessors operating at even higher clock speeds. Aside from going to more exotic types of heat removal, the power dissipated must be reduced without reducing clock speeds.
Three techniques are generally used to reduce power dissipation: reducing capacitive loading of internal nodes within the microprocessor, power supply voltage reduction, and selective clock speed reduction. The first approach is typically dependent on the dimensions of junctions and conductors of the fabrication process used to make the microprocessor and are not generally under the control of a circuit designer. The last two techniques may be used in combination. Because the power dissipation is related to the square of the power supply voltage, even a small reduction in power supply voltage makes a significant reduction in power dissipation. Since power dissipation is proportional to clock frequency, if the clock is removed or significantly slowed in portions of the microprocessor not being used at any given time, very little power is dissipated in those portions and the overall power dissipated is significantly reduced.
However, these power savings come at a cost. Power supply current can swing widely--from hundreds of milliamperes to over ten amperes with the microprocessor unable to tolerate more than a few percent change in voltage. Further, the change in current can occur in tens of nanoseconds and may change in magnitude with the instructions and data being processed. The power supply designed to supply the microprocessor must have a sufficiently low impedance and tight regulation to supply such dynamic power consumption. Moreover, if the power supply voltage is only a few volts (e.g., 3.3 or even 2 volts), the power supplies that can deliver over ten amperes at these voltages are very difficult to make and control and still operate efficiently.
To further complicate matters, the microprocessor may be powered at a different voltage than the rest of the integrated circuits in the computer. For example, the voltage available to power components in the computer is typically five volts with the microprocessor operating at three volts or so. Usually, a dedicated power supply for the microprocessor is placed in close proximity to the microprocessor and preferably on the same circuit board therewith. Thus, the power supply must be small and efficient. To meet these requirements, a small DC-to-DC switching power regulator is usually used.
Switching regulators are widely used in the DC-to-DC power supply market because they are generally efficient in terms of both power conversion as well as size. The typical kind of switching regulator used to convert a higher input voltage to a lower output voltage is known as "buck" regulator. Three kinds of feedback are generally used to control the operation of the regulator: voltage alone (with current limiting), voltage with peak current control, and voltage with average current control. See "Fueling the Megaprocessors--Empowering Dynamic Energy Management" by Bob Mammano, published by the Unitrode Corporation, 1996, pages 1-5 to 1-6 and incorporated herein by reference, describing these types of feedback as part of a buck switching regulator. For microprocessor applications, the voltage with average current control type of regulation is generally preferred over the other types for the described reasons. However, regulators using a lumped resistance in series with the output thereof for current sensing (both for peak current as well as average current control techniques) usually has significant power dissipation therein (e.g., one watt or more) at the higher output currents. (The resistance must be high enough to provide a sufficiently high voltage, usually tens of millivolts, to overcome input offset errors of the sense amplifier connected to the resistor at moderate output currents.) This reduces the overall efficiency of the regulator and reduces available margin for output voltage variation, as well as requiring a physically large resistor to handle the dissipated power. Further, the circuitry implementing the average current control technique is significantly more complicated than the circuitry of the other two techniques.